1. Field Of the Invention
The present invention relates to electrostatic copiers and particularly that class of copier which utilizes a developer mixture composed of a toner and carrier particles where the mixture is transported from a reservoir to an intermediate carrier by means of magnetic brushes.
2. Prior Art
In general this invention relates to devices for developing electrically charged images which images have been imposed on an intermediate carrier member by an electrostatic technique. Such devices are herein called copiers. Development of the images is through the use of a developer mixture made up of a toner and carrier particles. The developer mixture is transported from a developer mixture reservoir onto selected areas of the intermediate carrier in the outline of the desired image by means of rotating cylinders of the type known as magnetic brushes. Such devices are primarily found in copying machines and non-mechanical printers.
The generation of characters or images on a data carrier, for example, a paper sheet or web, by means of electrostatic copying is well known. See for example U.S. Pat. No. 3,099,856. In such devices electric charge latent images of the character to be reproduced are produced either electrographically or electrophotographically on an intermediate carrier, for example a drum which has a dielectric or photoelectric layer. These charge images are then developed using a toner to provide a toner image on the drum. The toner image is subsequently transferred at a transfer station to the paper or other data carrier. Thereafter the transferred toner images can be fused to the data carrier at a fixing station.
In copiers of this class, it is well known to utilize devices known as magnetic brushes for the development of the charge images on the intermediate carrier by the transfer of the toner to the image area. See for example U.S. Pat. No. 3,455,276. The magnetic brushes of the above mentioned type may consist of rotatable cylinders which have permanent magnets positioned interior thereof. The developer mixture which is used to develop the charge image on the intermediate carrier is a compound or admixture of toner and magnetic material carrier particles. The developer mixture is removed from a developer-mixture reservoir by means of a magnetic brush which passes through the reservoir and which thereafter transports the developer mixture to the intermediate carrier. In providing a path from the reservoir to the intermediate carrier for the developer mixture, one or more magnetic brushes may be arranged side by side with their internally positioned magnets being poled in such a manner that the developer mixture will be passed from one magnetic brush to the next adjacent magnetic brush.
By use of such magnetic brushes, the developer mixture will be conveyed past the intermediate character in such a fashion that the toner in the developer mixture can transfer to the intermediate carrier precisely at the location on the intermediate carrier where the charge images have been formed. The carrier particles and the toner not adhering to the intermediate carrier thereafter drop back to the reservoir.
In high performance units, for example nonmechanical high speed printers, the supply of the developer mixture to the intermediate carrier involves the transfer of a relatively high quantity of developer mixture. Only by transfer of a large quantity from the reservoir is it possible to properly develop the latent images on the intermediate carrier. This supply of large quantities of developer mixture can be achieved by use of several magnetic brushes, high transport speeds, strong magnets interior of the brushes and the like. It is important in such devices, however, that the developer quantity at the point of transfer between the intermediate carrier and the magnetic brush should be precisely metered. On one hand, the developer mixture should not be so thin in the transfer zone or development zone so as to prevent achievement of an adequate development of the charge image which would occur if insufficient toner is present at the nip area between the transfer magnetic brush and the intermediate carrier. On the other hand the mixture should not be present at the nip area in such a large degree as to be squashed between the brush and intermediate carrier since this will both cause wear of the surface of the intermediate carrier and prevent achievement of a clean background on the final print. Thus metering of the quantity of developer material at the nip area of toner transfer is important.